This invention relates to a process for making a fluid processing module such as a membrane filtration apparatus for effecting filtration of a liquid composition wherein a feed liquid is introduced into the apparatus and a filtrate stream and, optionally a retentate stream are removed from the apparatus. More particularly, this invention relates to a process for making a fluid processing module such as membrane filtration apparatus that are formed by indirect heat sealing of a polymeric composition.
For convenience, this invention will be described in detail with reference to a filtration module. However, it is to be understood that the process of this invention is equally applicable for making other fluid processing modules such as manifolds, heat exchangers, dialysers, desalters, degassers and the like. Prior to the present invention, liquids have been filtered within a plurality of filter modules that are stacked between manifolds or individually sealed to a manifold plate. Each module includes a one or more filter layers separated by appropriate spacer layers, such as screens, to permit liquid feed flow into the apparatus as well as filtrate flow from the apparatus. Filtration within the module can be conducted as a tangential flow filtration (TFF) process wherein incoming feed liquid is flowed tangentially over a membrane surface to form a retentate and a filtrate. Alternatively, filtration can be conducted as a dead end mode otherwise identified as normal flow filtration (NFF) wherein all incoming feed liquid is passed through a membrane filter with retention of solids and other debris on the membrane filter. In this latter mode only a filtrate is recovered.
At the present time, a filtrate stream is sealed from a feed stream within a membrane filtration apparatus by sealing techniques utilizing potting adhesives such as epoxies, urethanes or silicones, solvent bonding or direct heat sealing. In the case of a tangential flow filtration apparatus, a filtrate stream is sealed from a feed stream and a retentate stream. Adhesives are undesirable since they have limited chemical compatibility, are a source of significant extractable species, introduce process control difficulties, impose bond strength limitations, impose use temperature limitations and increase process cycle time. Direct heat sealing wherein a heating element contacts a material that flows to form a seal is undesirable since its use imposes a minimal limitation upon the thickness of the material being heat sealed. This results in a reduction of the number of layers that can be present in a given volume of the filtration module, thereby undesirably reducing the filtration capacity of the module. In addition, direct heat sealing is undesirable because it requires multiple steps, imposes material compatibility limitations, and typically utilizes a substrate to effect direct heat-sealing of filtration elements and can cause membrane damage. Solvent bonding is undesirable since solvents impose environmental issues and process variability while potentially useful polymers are limited by their solvation characteristics.
U.S. Pat. No. 5,429,742 discloses a filter cartridge comprising a thermoplastic frame into which are molded a plurality of filtration membranes. The thermoplastic frame is molded to provide fluid pathways that assure incoming fluid to be filtered to be passed through a membrane prior to removing filtered fluid from the filter cartridge. The frame is sufficiently thick so that fluid pathways to and from the membranes can be formed. Since adjacent membranes are separated by relatively thick spacer members, membrane area per unit volume of the filter cartridge is undesirably low.
Accordingly, it would be desirable to provide a method for making a fluid processing module such as a multilayer filtration apparatus which utilizes a plurality of filtration elements wherein the layers are appropriately sealed without the use of adhesive, solvent bonding or direct heat sealing. In addition, it would be desirable to provide a process for making a fluid processing module tangential flow containing a large number of layers such as filtration layers per volume of filtration apparatus which can be formed into a stack and which can be appropriately sealed to define liquid flow paths within the stack. Such a filtration apparatus would provide a high filtration capacity and would permit multiple uses of the apparatus.